Wave transmission system



June 1 2, 192s. Y 1,673,031

D. F. WHlTlNG WAVE TRANsMIssIN SYSTEM 'Patented June 12, 1928.

i y.UNITED STATES PATENT oFFIcEJ.

DONALD F. WHITING, OF PORT WASHINGTON, NEW YORK,` ASSIGNOR TO BELL TELE-PHONE LABORATORIES, INCORIORATED, OF NEW YORK, N. Y., A CORPORATION OFNEW YORK.-

WA-VE TRANSMISSION SYSTEM.

Application filed March 22, 1926. Serial No. 96,418.

This invention relates to wave transmission, and relates particularly totransmitting complex waves, such as waves representing speech` or music.

5. An object of the inventionis simplicity and efficiency intransmitting waves of a plurality of frequencies without unduedistortion, such as, for example, distortion created by generation ofwaves of other frequencies.

It 'is also an object of the invention to efficiently transmit complexwaves, or waves having components which are of different frequencies,such as, the complex waves instanced above, without undue distortioneither of the particular type just mentioned or other type, as forexample, the type of distortion due to transmission of components havingdifferent frequencies with different degrees of transmission eficiency.

It is also an object of the invention to couple the output of anelectron discharge device to a receiver circuit to efficiently transmitspeech or music waves without olistortion.

In high quality reproduction of speech and music it is necessary thatthe reproduced waves throughout a wide frequency range be essentially ofthe same relative amplitude as the original sound waves and also that nowaves of Afrequencies which were not originally present b'e introduced.Consequently it is necessary not only that the circuit used shoulduniformly transmit waves of all frequencies within the desired range butalso that no new fre uencies be produced. When electron disc argedevices are employed, it is therefore essential that they should not beoverloaded so as to 40 produce harmonic products of thefwavestransmitted. u

The wave spectrums of speech and music `contain ranges in theneighborhoodof 170 cycles per second and 64 cycles per secondrespectively, in which the energy level is normally several times higherthan that which obtains over the remaining portion of the spectrum. Theelectron tube circuits must therefore be designed so as not to over--load at these frequencies which impress the greatest amplitude onthe'grid, while at the same time amplifying waves throughout the entirespectrum substantially uniformly.

' As has been pointed out, for example, by

IV. J. Brown on page 289 of the March 1924 issue of the Journal'of theInstitution of Electrical Engineers, London, the maximum distortionlessoutput of a vacuum tube is obtained when the tube is operating into aload having an impedance about double that of the tube. However, it isoften necessary to use vacuum tubes in connection with output circuitshaving reactive impedances, such for example, .as loud speakingreceivers or transmission lines, and if this impedance ratio is set upfor the low frequencies, at which the energy level is highest, aconsiderable undesired loss due to mismatched impedances will Aoccur fat the higher f requencies at which the load impedance is greatlydifferent. If the impedance of the tube and load circuit be matched atsome intermediate frequency there results premature overloading of thetube at the lower frequencies and still a considerable transition lossat the higher; and even if an' equalizing arrangement be employed formatching the impedances throughout the larger portion of the range therestill results a premature overload at the lower frequencies.

In accordance with one specific aspect of this invention there isprovided a vcoupling means having such an impedance ratio atlowfrequencies that the imp'efancel into which the tube works ispreferably about double the impedance ofthe tube and having such a ratioat high frequencies that the impedance into which the tube works ispreferably about equal to its own impedance.

Other objects, aspects and features of the invention will be apparentfrom the following description of specific examples of systems utilizingfeatures of the invention, taken in connection with the claims and withthe drawings.

Figs. 1, 3, 4 and 5 show transformer arrangements according to thisinvention for coupling a tube to aload having an -inductive impedance,and Fig. 2 shows a transformer arrangement for cupling a tube to a loadhaving a capacity impedance.

Fig. 1 shows a vacuum-tube 7 having a cathode 8, a Grid 9 and plate 10.The cathode is heater in the -usual manner by current supplied from abattery 11.v A second batte-ry 12 is connected in the late'circuit forsupplying plate current. T e waves to in that range.

50,000 ohms at the higher.- In

'such as to give the maximum be amplified are impressed from the line 13through an input transformer 14 onto the grid circuit. The amplifiedwaves are impressed on an electromagnetic loud speakmg receiver 15. Twotransformers 16 and 17' having their primary windings 18 and 20connected in series to the plate circuit of transformer, to provideshunt lpaths for the high frequency currents.v Transformer 17 isdesigned to give good transmission at the higher frequencies and to havethe. proper impedance ratio to prevent transltlon loss To'aid in theunderstanding of the invention values typical of those which may befound in practice will be. assigned to the various circuit elements.Accordingly,y it may be assumed that. the vacuum tube has a plateimpedance of 4000 ohms and operates into a receiver having an impedancevarying from 2000 ohms -at the lower frequencies to be transmitted vtoapproximately such acircuit, transformers l16 and 17 should haveimped-8,000 4000 v 1 m and m respectiv e y. Although at the low frequenciesthe impedance into which the tube. works be such as to give the maximumenergy output from the tube for those frequencies that is possiblewithout introduction of distortion, and, on the other hand at the highifrequencies the impedance into which the tube works be ossible energyoutput from the tube, this di erence will not cause material distortionof the speech or music by causing the relative amplitudes of ance ratiosof the lowfrequencies andthe high frequencies, after reproduction, to bematerially' different from their relative amplitudes when they areimpressed on the system. The

' transmission loss, or loss'of gain, at the low frequencies that is dueto the use of the transformer ratio which Ygives the maximum possibledistortionless output from a tube for those frequencies instead `of themaxi-` mum possible output from the tube for those frequencies,is-.of'the order of one-half transmission unlt, which represents achange in the energy 'of sound not noticeable by the average ear. Thus,ordinarily'no compensation or adjustmentv for the system would berequired for the purpose of obtaining substantially uniform transmissioneiiiciency for the low andthe high frequencies because of thetransformer ratio inthe case of the low ondary winding 44.

- impedance ratio to frequencies being the ratio for maximum possibledistortionless output from the tube at those frequencies and the ratiofor the1 high frequencies being the ratio for maximum possible outputfrom the tube at the high frequencies. d

Fig. 2 shows a circuit similar to that of Fig.- 1 except that the tubeis operating into a loud speaking receiver having a capacity impedancesuch as for example a piezo-electric loud speaker of the type disclosedinthe application of A. Nicolson, Serial No. 683,643, filed December 31,1923. The transformers 25 and 26 are connected in series to' thecircuit. of Fig. 1 in the same manner as thetransformers 16 and 17.Transformer 25 is designed to have the proper impedance ratio to preventoverloading at the lower frequency, and its windings are shunted by thecondensers 27 and 28 in the same manner as transformer 16. Assuming inthis case that the same type of vacuum tube having a plate impedance of4000 ohms isused and that the impedance of the receiver varies from,50,000 ohms at the lower frequencies to be transmitted, to 2000 ohms atthe higher frequencies, transformer 25 should have an impedance ratio ofand transformer 26 should have an impedance ratio of Fig. 3 shows anamplifier of the pushpull type employing two tubes 37 and 38 connected`in the usual manner t0 receive Waves from -an input transformer 34. Theplate circuits of the tubes are connected to the primary windings 39,40, 4l and 42 and are 1n series between the plates 29 and 30 of thetubes 37 and 38respcctively. l/Vindings 39 and 42 together with theircorrespending secondary windings 43 and 45 may be mounted on a commoncore while the windings 40 and -41 and their secondary winding 44 may bemounted. on a separate core. A connection is made from the cathodes 35and 36 of the tubes 37 and 38 to the plate battery 22 tothe commonconnection of windings 40 and -41 so that the'output transformerarrangement is symmetrical with respect to the two tubes. The secondarywindings 43, 44 and 45 are connected in series to the load'circuit 46which may be a loud speaking receiver such as that shown in Fig. 1 forexample. vThe transformer 40, 41, 44 is designed to give the properimpedance ratio at low frequencies.- As condenser 47 is connected inshunt to the primary windings40 and 41, similarly a condenser 48 isconnected in shunt to the sec- These condensers 'provide paths for thehigh frequency currents. The couplings 39 and 43 and 42 and 45 aredesigned to give good transmission at' the h1 gher frequencies and tohave the proper prevent transition loss plings 89, 42 and 43, 45should'have anl 8000 50000 ance ratio of the transformer 40, 41 and 44The circuit shown in Fig. 4 employs a single vacuum tube as in Fig. lbutthe transformers 16 and 17 instead of being connected in seriesareconnected in parallel between the plate circuit of the tube and theload circuit 46. Condensers 48 and 49 are connected in series with theprimary 20 and secondary 21, respectively of the trans former 1T toprovide high impedance for the low frequency currents. While inductancecoil 51 is connected in series with the secondary winding 19 of thetransformer 16 to provide a high impedance for the high frequencycurrents. A separate inductance for this purpose need notvnecessarily-be used but instead the transformer winding may be designedto have high leakage inductanee. If the same type-of tube and receiveris used as was assumed for the circuit of Fig. 1, the impedance ratiosof the transformers should be the same as was used in connection withthe transformers of that circuit.

Fig. .5 shows a circuit employing tubes arranged in push-pull relationas in Fig. 3 eximpedance ratio of while the impedshould be `cept that inthis case the transformers are connected in parallel rather than inseries. A condenser 52 is connected between the windings 39 and 42 andcondenser 53 between the windings 43 and 45 to provide a high impedanceto low frequency currents. Inductance coils 54 and 55 are connected inseries with the secondary winding 44 to provide a high impedance forhigh frequency currents. Two coils may be used to keep the circuitbalanced or this result may also be obtained if the two windings 54 and55 are wound on the same core. In the low frequency transformer thebalanced. arrangement is maintained by connecting the condensers between4the halves of the primary and secondary windings. Assuming the samecircuit conditions as was assumed for Fig. 3, the transformers `shouldhave the same impedance ratios as was specified for that circuit.

In the description Vof the circuits of Figs. 4 and 5, the 'referencenumerals which have been used to designate the circuit elements are thesame as those by which similar circuit elements have been designated inthe circuit of Figs. l and 3 respectively.

While several embodiments of the invention have been shown theymay bevariously modiiied without departing from the spirit of the inventionasdeiined in the appended claims. i

What is claimed is 1. 1n a circuit for the transmission of waves of aband of frequencies having waves in one portion normally at a higherenergy level than waves in the remainder of the band, an velettrondischarge device,' an out* put circuit for said device, transformerequal to substantially twice the impedance ratio of the coupled circuitsat said frequency so that sai-d device works into an iinpedaiicesubstantially equal to twice its output impedance, said transformermeans havingat frequencies in the remainder of said band an impedanceratio substantially equal to the impedance ratio of said coupledcircuits.

2. In a circuit for the transmission of waves-in the audible frequencyrange, an electron discharge device, an output circuit for said devicehaving a reactive impedance, transformernieans for coupling said deviceto said circuit, said means being responsive to waves of frequencies invthe lower portion of the audible frequency range and having animpedance ratio equal to substantially twice the ratio of the impedancesof the coupled circuits at said frequencies so that said device worksinto an impedance substantially equal Lto twice its output impedance,and transformer means for coupling said device to said circuit, saidmeans being responsive to waves of frequencies in the upper portion ofthe audible frequency range and having an impedance ratio substantiallyVequal to the impedance ratio of the coupled circuits at saidfrequencies.

3. The combination according to claim 2 in which each of the transformermeans have primary and secondary windings connected in series to saidelectron discharge device and said circuit, respectively, andAcondensers connected in shunt to both the primary and secondarywindings of said first transformer means.

4. In a circuit for the transmission of waves in the audible frequencyrange, a vacuum tube amplifier, an output circuit for said amplifierhaving a reactive impedance, transformers for coupling said amplifier tosaid output circuit, one of said transformers having primary andsecondary windings divided into halves and the other of saidtransformers-having primary and secondary windings connected between thetwo halves of the windings of said first transformer and condensersconnected in shunt to the impedances of said amplifier and saidoutputcircuit at said -frequency, and said other transformer havingat'frequencies in the lower portion of the audible frequency range, animpedance ratio substantiallyequal'to twice the ratio of the' impedancesof said lamplifier and said output circuit.

5.' In combination, a translating device, a load whose impedance isrelativelylow at one portion of the frequency range to be ltransmittedand relatively high at another portion to be transmitted, anda pluralityof transformers, certain of said 'transformers serving to transmit thefirst mentioned portion of said frequency range and having an impedanceratio such as to give substantially maximum distortionless output fromsaid device to said load, and certain other of said transformers servingto transmit the other mentioned portion of said frequency range andhaving an impedance ratio such as to give substantially maximum outputfrom said device to said load.

In. witness whereof, I hereunto subscribe my name this 18 day of MarchA. D., 1926.

` DONALD F. WHITING.

